Autoimmune thrombocytopenia is often due to antibodies reactive with platelet glycoprotein (GP) complexes, particularly GPIIb/IIIa. Autoepitopes are often divalent cation-dependent. The investigators will focus on two research areas: autoantibody characterization and antigenic localization. Autoantibody characterization. They will: 1) Demonstrate the presence of platelet-associated and plasma antibodies and focus on antibodies against GPIIb/IIIa, since these are most common; 2) Evaluate antibody clonality by examining light chain and IgG subtype characteristics and 3) Study autoantibody binding to megakaryocytes in vitro and determine if bound antibody effects megakaryocyte maturation, activates complement or induces phagocytosis. Antigen localization. They will: 1) Determine if rabbit antibody to the GPIIb/IIIa calcium-bindings sites will inhibit autoantibody binding; 2) Test autoantibody binding to a series of large recombinant peptides, spanning glycoproteins Iib and IIIa. They will insert specific PCR-generated cDNA fragments into a vector which results in their expression by E. Coli as fusion proteins with a 6X-histidine tag which can then be affinity-purified using a nickel nitrilo-tricacetic acid resin column. Antibodies which bind to one of the large peptides will be studied further by determining binding patterns to smaller peptides within this region; 3) Evaluate epitopes, not identified by this technique, with random peptide libraries and GPIIb/IIIa chimeras. Antibodies are incubated with fUSE5 filamentous phage expressing several million random peptide sequences. Phage expressing sequences which bind to antibodies are separated from non-specific phage by serial 'planning' with staphylococcal-A protein agarose followed by acid elution. The sequence of the specific peptide(s) is determined. In addition, they will develop human/xenopus and human/avian GPIIb/IIIa chimeras which they will use to examine the binding patterns of human antiplatelet autoantibodies. Results of these studies will be correlated with the patients' clinical course and thrombopoietin levels. In addition, the effect of immune thrombocytopenia on the patients' quality of life will be studied.